Ras-independent activation of Rel-family transcription factors by UVB and TPA in cultured keratinocytes.

UV irradiation of mammalian cells results in the activation of transcription factors which mediate induction of early response genes designed to repair and minimise the damage sustained by the cell. Evidence from studies in HeLa cells suggest that UVC regulates NF-kappa B activity via tyrosine kinases and activation of Ras and Raf kinase. In this study we have used a previously characterized TPA-responsive element (VLTRE) that binds Rel/NF-kappa B proteins and a Ras-responsive element (B10 RRE) to analyse the signalling pathway in UVB-stimulated gene transcription in cultured keratinocytes. We demonstrate that the tumour promoters TPA and UVB use different signalling intermediates to activate different sets of Rel/NF-kappa B proteins. UVB transactivation is independent of PKC activity but dependent on tyrosine kinase activity where was TPA stimulation requires PKC but not tyrosine kinase activity. Furthermore, neither UVB- nor TPA-transactivation is mediated through p21 Ras but both stimuli are dependent on a functional Raf protein. A constitutively active Raf-1 kinase however, was unable to induce transactivation through VLTRE. Thus, Raf has an essential but permissive role in UVB activation of Rel proteins. These findings demonstrate that keratinocytes contain a novel Ras-independent pathway for induction of Rel mediated transcription.